Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recovery technology for producing heavy crude oil and bitumen. The gravity drainage idea was originally conceived by Dr. Roger Butler around 1969, and field tested in 1980 at Cold Lake, Alberta, which featured one of the first horizontal wells in the industry with vertical injectors. The latter were established to be inefficient, resulting in the first test of twin (horizontal) well SAGD in the Athabasca Oil Sands, which proved the feasibility of the concept, briefly achieving positive cash flow in 1992 at a production rate of about 2000 bbl/day from 3 well pairs.
In the SAGD process today, two parallel horizontal oil wells are drilled in the formation, one about 4 to 6 meters above the other. Steam in injected via the upper well, possibly mixed with solvents, and the lower well collects the heated crude oil or bitumen that flows out of the formation, along with any water from the condensation of injected steam. The basis of the process is that the injected steam forms a “steam chamber” that grows vertically and horizontally in the formation. The heat from the steam reduces the viscosity of the heavy crude oil or bitumen, which allows it to flow by gravity down into the lower wellbore. The steam and gases rise because of their low density compared to the heavy crude oil below, ensuring that steam is not produced at the lower production well. The gases released, which include methane, carbon dioxide, and usually some hydrogen sulfide, tend to rise in the steam chamber, filling the void space left by the oil and, to a certain extent, forming an insulating heat blanket above the steam. Oil and water flow is by a countercurrent, gravity driven drainage into the lower well bore. The condensed water and crude oil or bitumen is recovered to the surface by pumps such as progressive cavity pumps that work well for moving high-viscosity fluids with suspended solids.
Operating the injection and production wells at approximately reservoir pressure eliminates the instability problems that plague all high-pressure steam processes and SAGD produces a smooth, even production that can be as high as 70% to 80% of oil in place in suitable reservoirs. The process is relatively insensitive to shale streaks and other vertical barriers to steam and fluid flow because, as the rock is heated, differential thermal expansion causes fractures in it, allowing steam and fluids to flow through. This allows recovery rates of 60% to 70% of oil in place, even in formations with many thin shale barriers.
Thermally, SAGD is twice as efficient as the older cyclic steam stimulation (CSS) process, and it results in far fewer wells being damaged by high pressure. Combined with the higher oil recovery rates achieved, this means that SAGD is much more economic than pressure-driven steam process where the reservoir is reasonably thick.
This technology is now being increasingly exploited due to increased oil prices. While traditional drilling methods were prevalent up until the 1990s, high crude prices of the 21st Century are encouraging more unconventional methods (such as SAGD) to extract crude oil. The Canadian oil sands have many SAGD projects in progress, since this region is home of one of the largest deposits of bitumen in the world.
As in all thermal recovery processes, the cost of steam generation is a major part of the cost of oil production. Historically, natural gas has been used as a fuel for Canadian oil sands projects, due to the presence of large stranded gas reserves in the oil sands area. However, with the building of natural gas pipelines to outside markets in Canada and the United States, the price of gas has become an important consideration. The fact that natural gas production in Canada has peaked and is now declining is also a problem. Other sources of generating heat are under consideration, notably gasification of the heavy fractions of the produced bitumen to produce syngas, using the nearby (and massive) deposits of coal, or even building nuclear reactors to produce the heat.
In addition to the operating costs of generating steam, a source of large amounts of fresh and/or brackish water and large water re-cycling facilities are required in order to create the steam for the SAGD process. Thus, lack of water and competing demands for water may also be a constraint on development of SAGD use. Further, since SAGD relies upon gravity drainage, the reservoirs must be comparatively thick and homogeneous, and thus SAGD is not suitable for all heavy-oil production areas.
Alternative enhanced oil recovery mechanisms include VAPEX (for Vapor Extraction), Electro-Thermal Dynamic Stripping Process (ET-DSP), and ISC (for In Situ Combustion). VAPEX uses solvents instead of steam to displace oil and reduce its viscosity. ET-DSP is a patented process that uses electricity to heat oil sands deposits to mobilize bitumen allowing production using simple vertical wells. ISC uses oxygen to generate heat (by burning some amount of the oil reserve) that diminishes oil viscosity and also produces carbon dioxide. One ISC approach is called THAI for Toe to Heel Air Injection.
In most steam assisted gravity drainage (SAGD) operations, the SAGD steam is generated at a central processing facility (CPF) and conveyed to the wellpads, where it is injected into the SAGD reservoirs. An alternate approach is to locate the steam generating devices at the wellpads and convey the required water, fuel, and oxidant to the steam generators from the CPF. One example of a wellpad steam generator is the Direct Steam Generator (DSG) concept, where fuel is burned with oxygen in the presence of water to produce steam/CO2 for SAGD.
The performance of SAGD wellpad steam generators such as Direct Steam Generators (DSGs) can be enhanced by preheating the feedwater with waste heat from SAGD produced fluids. The conventional approach is to perform the feedwater preheating at the central processing facility. However, heat losses from the hot streams conveyed between the pads and the CPF will reduce the maximum attainable preheat temperature. Ideally a wellpad steam generator can solve this temperature drop problem, but no wellpad steam generator such as DSGs have been commercially deployed yet.
Direct Steam Generators are newly developed devices that can generate steam on the wellpad rather than at the central processing facility. The small footprint of a DSG may be especially favorable in view of the limited space at the wellpad. By implementing these on-site DSGs, energies could be conserved greatly due to the reduction of heat losses during steam transmission. However, further improvements can still be obtained.
What is needed in the art are improved SAGD methods that further reduce the cost and improve the efficiency of SAGD and related methods of oil recovery.